ENERGY STORAGE AND AUXILIARY HEAT

Since effective sunshine occurs only about 5 to 6 hours per day (in temperate latitudes) and

Figure 15-8.-Schematic of potable hot-water heating systems, using solar storage (tempering) tank ahead of the conventional fueled or electric service water heater. since heating and hot-water loads occur up to 24 hours a day, some type of energy storage system is needed when using solar energy.

Practical experience in the industry, as well as computer simulations and experiments, has

Figure 15-9.-Typical DHW installation.

resulted in rules of thumb for storage sizing. These guidelines provide storage sizes for which the performance and cost of active solar systems are optimized and relatively insensitive to changes within the range indicated.

Water Systems

Since water has a specific heat of 1 Btu/1b-F, then 15 pounds of water storage is needed per square foot of collector or 1.8 gallons of storage is needed for each square foot of collector.

Air Systems

Since rock has a specific heat of 0.21 Btu/1bF and rock densities typically contain 20 to 40 percent voids, then the optimum storage size is 0.8 ft3 per square foot of collector. Storage volumes in this range store the equivalent overnight of 1 full day of heating. A typical domestic hot-water system is shown in figure 15-8. The use of two tanks ensures that when hot water from the first (tempering) tank is available, the auxiliary heat does not come on; also less total fuel is used to bring the smaller second tank up to temperature. Single-tank arrangements are not recommended because they frequently activate the heating element every time there is a draw of water, rather than wait for the solar collectors to provide additional heated water. The two-tank arrangement (fig. 15-9) avoids this control problem. Two-tank arrangements are suited to retrofits since the second tank (the water heater) is already there. A variation would be to use a heat exchanger (copper coil) (fig. 15-10) in the tempering tank collector loop for freeze protection. The tempering tank then becomes an inexpensive unpressurized tank.

Another method of heat storage in air systems is latent heat storage. Latent heat is stored in a material as it changes state from a solid to a liquid. Materials that have melting points near the temperatures supplied by solar collectors store heat as they melt and release it as they resolidify. The two materials that have received the most attention are salt hydrates and paraffins.

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